How Do You Achieve Realistic Spinning for Exhaust Fans?
There are a couple of ways which you can fake the effect for a fan blade spinning fast. One would be to use or create a texture which has the original appearance of a fast spinning blade and then jut animate that.

Another way and more than likely what you are looking for is to use a vector blur node in compositing to add the blur effect to the blades.

Here is a link for how this can be used.

Vector (Motion) Blur Node

You can also use the motion blur effect located in the render tab to create motion blur for the animation. Here is an example for how something like this can be done.

Blender Tutorial - How to Create Motion Blur For The Wheels